He said - She said on acceptable compression results - is it in the manual or?

[WA-Newb]

After researching what may be a good compression result on an older Evinrude I am left with more questions.

1. Do some years of outboards actually come from factory with lower compression then other years? With same HP that is.

2. Does the acceptable PSI depend greatly on the brand and model of the motor or is general across the board?

3. Will a normal shop manual state what the reading should be for that given motor?
If so, is there a way to calculate what is acceptable after X number of years.

My motor is around 92 PSI on all 6 cylinders..that was done cold I believe but am planning on re-doing it soon when it's warmed up.
It is up for sale and half the people that have called on it say the compression is good..and the other half say it's way too low.
I have no idea what to tell them and never want to steer someone wrong...I am hoping to find actual info from factory somehow that will help this topic either way.

Thank you.

 

[BonairII]

Verify those numbers with another compression gauge. Having all 6 cyls reading low AND equal is suspect IMO.

 

[Bob_VT]

The V-6 motors are lower compression then the V4 motors and 3 cylinder motors. Do a good decarb and the numbers will probably come up a bit but 90 lbs + on a V-6 is fine.

 

[F_R]

Manuals do not specify compression values. However, it would be unusual for all 6 cylinders to be "low" but equal. For that reason, I'd give it a passing grade.

 

[HighTrim]

All models vary slightly regarding acceptable compression. The new from factory compression readings were never published, but we can get close from testing very low hour motors. What is acceptable on a 60s 9.5hp motor (low compression motor), would not be acceptable on a big twin of similar vintage.

what model do you have specifically?



Im with Frank, being all equal is a good sign. All gauges vary slightly. The 6 cylinders dont seem to be real high compression powerheads. A good decarb would bring it up some, see Top Secret Files, but I wouldnt be worried about it.

 

[vistacruiser]

I can tell you this much, my 1990 GT200 reads 92psi cold with my gauge, in 5 of the 6, and does about as well as I could expect pushing my 20' BR, (glastron, heavy boat) into the low 50's at 5600 rpm. (19p, 4 blade SS). When I bought the boat used, all 6 were at 92 psi, then 5 years ago I had a cylinder seize up at WOT after which I had zero compression in that hole. Honing with a 3 stone and a new standard piston and rings brought it back up to 85 psi, despite some bad scoring. I am curious to know what a hot reading would be, never tried it. But 92 seams ok to me.

 

[emdsapmgr]

You did not state the year/model of the engine. That would be helpful when trying to correlate your numbers. Compression on V6 engines depends on model and year. The early (1976 to around 1983-1985) crossflow high compression heads would yield anywhere from 110 to 120+, depending on heads. In the mid 80's OMC increased the head area, effectively lowering compressions on all V4 and V6 engines. 90-100 was common. The later big bore 90 degree loopers run this same lower compression right out of the box from the factory. The early small bore 60 degree loopers with the high compresson Intruder/Faststrike heads did run some high compression numbers like the old crossflows.